1. Technical Field
This invention relates generally to the field of controls for commercial refrigeration equipment and, more particularly, to demand defrost controllers for refrigerated display cases of the type commonly found in supermarkets.
2. Background Information
During the operation of refrigerated fixtures, such as display cases, frost accumulation occurs on the evaporator coils, because the operating temperature of the evaporator is typically below the freezing point of water. Defrost cycles are necessary to remove the accumulated frost and restore air flow through the evaporator coils. If not removed, the frost build-up inhibits heat transfer at the evaporator which in many instances prevents the evaporator from maintaining the fixture at the desired temperature. In severe situations, the lack of heat transfer at the evaporator will result in incomplete evaporation of the refrigerant which can lead to damage to the compressor.
Evaporator defrost is generally accomplished by one of the following methods:
Off-cycle--In this method the refrigeration is shut off to the fixture and the frost is allowed to melt. This technique is generally used in situations where the expected frost build-up is small and lengthy periods of no refrigeration will not cause damage to the refrigerated product.
Electric--Electric heaters are employed at the evaporator to melt the frost. Air is blown over the heater and then through the evaporator for this purpose. Defrost durations are short in most applications, but electric power consumption can be significant, especially in applications such as supermarkets where the amount of refrigerated fixtures that must be defrosted is large.
Air defrost--Certain display case manufacturers offer an air defrost which consists of redirecting the case air curtain flow such that warm, ambient air is passed through the evaporator, melting the frost.
Hot gas--With hot gas defrost, hot discharge gas from the compressors is piped through the evaporator where it condenses and melts the frost accumulation. At present, hot gas defrost is employed with multiple parallel compressor racks because a source for the hot gas used in the defrost is needed. With parallel compressor racks, multiple refrigerated circuits are normally piped to them which allows defrost of one of these circuits while the remainder of the circuits are kept refrigerated. These refrigerated circuits are the source of the hot gas for the defrost. Hot gas defrost is considered the quickest and least energy consuming type of defrost, but it is also the most complicated in terms of piping and control required.
The most common form of defrost now employed for commercial refrigeration is electric. Hot gas defrost is used exclusively with multiple compressor racks and is normally found in newer supermarkets where more sophisticated refrigeration equipment has been installed.
In terms of energy consumption, electric defrost is the greatest power consumer for defrost since electric heaters are used directly in the process. Added energy consumption is also experienced by the refrigeration system compressors due to the need to remove any excess heat introduced into the fixture during defrost and to return the fixture to the desired operating temperature. In the case of the use of hot gas defrost, energy consumption is greatly reduced since no electric heaters are employed, and the amount of heat added to the case for the defrost is minimized. Added compressor power consumption is incurred, however, after the defrost, for the purpose of lowering the temperature of the fixture to the desired level.
Presently, defrosts for refrigerated fixtures, such as display cases, are most commonly controlled by the use of a time clock that schedules the defrosts on a regular basis. The time interval between defrosts for a particular display case is set based upon operation at design conditions, which are typically 75.degree. Fahrenheit, dry bulb temperature, and 55% relative humidity. However, the ambient humidity in which the refrigerated fixture is operated is often less than the design value. At these times, defrost is not required as often and unnecessary defrosts are incurred due to the inflexibility of the time clock control. The penalty associated with unnecessary defrosts is added electric consumption by electric defrost heaters, where employed, and by the refrigeration compressors which must provide added refrigeration to return a display case to the desired operating temperature immediately after defrost.
Modifications have been made to the conventional defrost time clocks to allow them to adjust to some extent to actual operating conditions. One such timer changes the time between defrosts based upon the time required to complete the previous defrost. Another type of adaptive controller varies the time between defrosts based upon the ambient humidity in the store. A shortcoming of this latter type of controller is that no attempt is made to assess defrost requirements on an individual case basis. Unnecessary defrost may still occur because the number of defrosts per case has not changed.
To eliminate unnecessary defrosts, attempts have also been made in the past to develop demand defrost systems which determine through measurement when defrost is required. The various types of demand defrost systems that have been tried include the following:
Frost sensors--An optically based sensor is located at the evaporator for the purpose of detecting frost build-up. When the frost reaches a preset thickness, the defrost is initiated. Several sensors of this type have been marketed with only limited success. The primary problems incurred have been that the sensors were susceptible to triggering unnecessary defrosts and tended to be maintenance intensive. Because the sensor was located at the display case evaporator, replacement of a failed sensor was very difficult, often requiring removal of refrigerated product from the case. For this reason the tendency was to replace the failed sensor with a time clock.
Temperature sensors--A demand defrost controller was developed and introduced by Honeywell Corporation (reference U.S. Pat. No. 3,777,505). The controller employed two temperature sensors located adjacent the inlet and outlet, respectively, of the display case evaporator. Defrost was initiated when the temperature difference between the two sensors reached a set point. This controller did not gain wide acceptance because it was prone to triggering defrost unnecessarily. Like the frost sensors, the temperature sensors were located at the display case evaporator, making them difficult to service.
Pressure sensors--The change in pressure drop on the air side of the evaporator as frost deposits on the surface represents a possible method of determining when defrost is required. The size of this pressure difference is relatively small, on the order of hundredths of inches WC, which is difficult to measure accurately. For this reason, the cost of pressure sensors is prohibitively high to make them attractive for defrost control.
In general, previous attempts to employ various measurements, such as frost thickness, temperature, and air pressure, for demand defrost have met with only marginal success. The reasons for this include the following:
sensor failure which can occur occasionally and either fail to initiate defrost when needed or trigger unnecessary defrost;
sensor readings are misinterpreted by the controller, initiating unnecessary defrost. In this instance, activity at the display case such as stocking, cleaning, etc. has caused a sensor reading similar to that produced when defrost is required.
Failure of the defrost controller for the above reasons often lead to service calls for the refrigeration equipment, loss of refrigerated product, etc. Accordingly, at present, no demand defrost control system has gained wide spread acceptance.
A need thus persists for a cost effective, energy saving, demand defrost controller which affords improved performance, reliability and maintainability in governing defrost frequency of a display case.